Type IV collagen is a distinctive glycoprotein which occurs almost exclusively in basement membranes, structures which are found in the basal surface of many cell types, including vascular endothelial cells, epithelial cells, etc. Type IV collagen is a major component of basement membranes. It differs from interstitial collagens. See New Trends in Basement Membrane Research, K. Kuehn et al., eds., Raven Press, N.Y., at pp. 57-67 (1982). Type IV collagen has a molecular weight (MW) of about 500,000 and consists of three polypeptide chains: two .alpha.1 (MW 185,000) chains and one .alpha.2 (MW 170,000) chain. Type IV collagen has two major proteolytic domains: a large, globular, non-collagenous, NCl domain and another major triple-helical collagenous domain. The latter domain is interrupted by non-collagenous sequences of variable length. A diagrammatic representation of the type IV collagen molecule is shown in FIG. 1. It is a complex and multidomain protein with different biological activities residing in different domains.
Type IV collagen self-assembles to polymeric structures which constitute the supportive frame of basement membranes. Various other macromolecular components bind to type IV collagen, such as: laminin, entactin/nidogen and heparin sulfate proteoglycan. An additional function of type IV collagen is to mediate cell binding. A variety of cell types specifically adhere and spread onto type IV collagen-coated substrata. See J. C. Murray et al., J. Cell Biol., 80, 197-202 (1979); M. Aumailley et al., J. Cell Biol., 103, 1569-1576 (1986); T. J. Herbst et al., J. Cell Biol. 106, 1365-1373 (1988). Various cell surface proteins, a 47 kD protein [M. Kurkinen et al., J. Biol. Chem., 259, 5915-5922 (1984)], a 70 kD protein [S. P. Sugrue, J. Biol. Chem., 262, 3338-3343 (1987)] and members of the superfamily of integrins [K. J. Tomaselli et al., J. Cell Biol., 105, 2347-2358 (1987)], have been reported to mediate cell binding to type IV collagen.
The variety of functions of type IV collagen suggests that this glycoprotein is important in many diverse and clinically relevant processes such as cell attachment and migration in wound healing, tumor cell invasion and metastasis, diabetic microangiopathy, vascular hypertrophy due to hypertension and several kidney diseases such as diabetic nephropathy and nephrotic syndromes of variable etiology. For example, in Goodpasture's syndrome, a disease characterized by hemoptysis and hematuria due to alveolitis and nephritis, respectively, an antibody to the major noncollagenous NCl domain of type IV collagen is found in the serum of all Goodpasture's patients. Another hereditary kidney disease, Alport's familial nephritis, is apparently due to a genetic defect of the NCl domain of type IV collagen. In addition, in diabetes mellitus, intact type IV collagen, as well as the triple helix-rich domain, are chemically modified and functionally impaired by the increased amounts of glucose present in the plasma and in the immediate vicinity of the basement membranes, i.e., in the extracellular matrix.
In order to better understand the pathophysiology of these processes at a molecular level, there is a need to try to assign at least several of the above-mentioned biological activities of type IV collagen to the specific proteolytic domains (i.e., NCl, triple helix-rich domains) or oligopeptides of type IV collagen. If this can be achieved, it will be possible to synthesize small peptides which can provide the basis for important pharmaceutical compositions.